Cement dispersants for improving concrete durability, ensuring concrete strength, and reducing the unit water content of concrete are an essential chemical admixture in today's concrete production. Among the cement dispersants, a high-range AE water reducing agent that is predominantly composed of a polycarboxylic acid polymer is excellent in an effect of reducing a unit water content, can achieve desired consistency during transportation from a manufacturing site of ready-mixed concrete to a job site to ensure excellent construction workability (high slump retention or decreased slump loss), and therefore is especially excellent in an effect of reducing construction defects. This allows concrete products factories to produce concrete products with less unit water content, resulting in increased production of high-strength precast components.
Along with the technical development in high-range AE water reducing agents, construction techniques and construction methods using the newly developed cement dispersants have been established, which has led to new problems and requirements to be overcome. Since the emergence of fundamental cement dispersants including a polycarboxylic acid polymer, such as one disclosed in Patent Document 1, in order to solve newly occurred problems and satisfy new requirements, polycarboxylic acid polymers having various molecular weights and various structures have been studied and have been developed for use in a cement dispersant.
Various suggestions have been made, including one to seek improvement in slump retention by controlling the molecular weight of the polycarboxylic acid polymer to fall within the suitable range (Patent Document 2), one to seek improvement in efficiency at the work site and in labor savings by reducing the degradation effect with time on fluidity (slump loss) and facilitating release from a formwork and early strength development in the resulting concrete by way of providing a polycarboxylic acid polymer the monomer composition of which is controlled to have a combination of monomers with different grafted side chains (the number of addition moles of ethylene oxide: 5 to 25, 40 to 109) (Patent Document 3), and one to seek exhibition of the fluidity and strength development in the resulting concrete by providing a polycarboxylic acid polymer that includes a polymer structure derived from a carboxylic acid monomer containing an alkoxy group or a hydroxyalkoxy group (Patent Document 4). Another suggestion that has been made is to provide a cement dispersant having high passability and fluidity and being able to develop the strength of the resulting concrete by controlling the molecular weight of the polycarboxylic acid polymer to be not lower than 5,000 and lower than 10,000 and the value of the weight average molecular weight/number average molecular weight to be not lower than 1.0 and not higher than 1.5 (Patent Document 5).
As described above, controlling the molecular weight and the structure of a polycarboxylic acid polymer in a cement dispersant significantly changes the work environment in the concrete production site and the construction site. Therefore, accuracy in polymerization of the polycarboxylic acid polymer has long been desired to be further improved in order to fully meet increasing needs.
Use of a polycarboxylic acid polymer recently started to cause various troubles and raise problems to be solved in the expectation for further improvement thereof. For example, it presents a challenge to reduce variation in the performance of a cement water-reducing agent due to temperature changes. This variation causes a problem that, during the winter months, the performance such as the water-reducing ability develops so slowly that the fluidity of concrete is excessive when the concrete arrives at the construction site even though the fluidity has been appropriately adjusted while kneaded in a ready-mixed concrete factory, which causes uneven placement including segregation at the time of construction, resulting in troubles such as nonuniformity in concrete strength. On the other hand, during the summer months, increased temperatures facilitate cement hydration to make it difficult to ensure the consistency for a desired period of time, and therefore a cement water-reducing agent that can achieve desired consistency to be retained for a prolonged period of time is desired.
A polycarboxylic acid polymer has a foaming property, which is less effective than that of a conventional naphthalene-based cement water-reducing agent, but nonetheless presents concerns about alteration in the air content of concrete and about the impact on the finished surface (hereinafter, called a pockmarked surface). Both of these are serious problems to be solved regarding the quality of hardened concrete because the air content of concrete is known to be associated with concrete strength and therefore a change in the air content directly involves degradation in the strength, and because the impact on the finished surface, which is subjected to neutralization and salt damage, is associated with concrete durability.